Gemologist Guide To Identifying Benitoite

Benitoite was discovered in 1907 and became the state gem of California in 1985. Initially, the opinions about the gem’s identity ranged from Spinel to Volcanic Glass, and because of the particular blue color, Sapphire. However, the gemstone’s birefringence and dispersion values helped distinguish it from Sapphire.

 

How to Identify Benitoite Through Testing

There are various ways to identify rocks, minerals, crystals, and gemstones, but we will use a method I learned while attending the Gemological Institute of America.  If you’ve learned a unique way to identify gems, please share it with us.

Let’s look deeper into how to identify Benitoite like a pro.

 

 

Visual Inspection

The visual inspection starts with what form of Benitoite you have.  The questions below are relatively easy to answer, but each type will have its own process for identifying them.

 

Is it a cabochon? A cabochon should have a medium to high polish with little to no pitting but these will be rare or uncommon. Most of this material will be faceted or in specimen form but you could come across a cabochon cut by a lapidary artist for their own collection. Don’t expect to see Benitoite cabochons at your jewelry store or gem show.

 

Is it faceted? If you have a faceted piece of Benitoite, it should be transparent and the biggest giveaway will be the dispersion. If you see rich blue color with a high dispersion then the stone is telling you it’s probably Benitoite. There is one caveat and that is the weight. If the stone weighs 4 or more carats then you might want to rethink it. Most of the faceted Benitoite on the market will be small and hard to find.

 

 

Is it a specimen? Benitoite is found in different forms. You’ll better identify these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Benitoite displays when it’s a specimen.

• Benitoite crystals typically grow from white host rocks of Natrolite over Schist.
• Some specimens contain a couple of larger crystals, while others have several small bits.
• The shades of blue vary from light to dark.

 

Is it tumbled? Very uncommon to find tumbled Benitoite.

 

Physical Properties of Benitoite

Let’s take a look at the physical properties of Benitoite. Knowing what to look for will help you more easily identify what you’re looking at.

 

Color: Various shades of Blue, including Sapphire Blue, White to Colorless, Rare: Pink

Clarity / Transparency: Transparent, Translucent

Luster: Vitreous

Cleavage: Poor/Indistinct – Poor on {1011}

Fracture: Conchoidal

 

The Streak Test

This is a destructive test, so you must ensure that you can damage the specimen or stone if you choose to use this method.  You won’t use destructive tests once you’ve developed robust knowledge in identifying rocks and minerals.

A mineral streak test is when you scrape the stone against a harder surface to see what color remains. Benitoite always produces a white streak.

Tumbled specimens are tested by scraping samples across a piece of ungalvanized porcelain, typically known as a streak plate.

 

Magnet Test

Benitoite is slightly magnetic due to its iron content. However, none of the other materials within the gems are.

 

Hardness Test

I don’t recommend actively testing the hardness of a stone because it’s destructive in nature and doesn’t really provide a definite answer to what type of stone it is.  Benitoite has a hardness of 6 to 6.5 on the Mohs hardness scale.

 

Refractive Index Test

Determining the refractive index, or RI, as it’s referred to by gemologists for Benitoite, is relatively straightforward. Still, you’ll need a specific piece of test equipment and the RI fluid to go with it.  Before you place the stone on the refractometer, you want to make sure you have a flat, somewhat polished surface to take a reading.

 

Benitoite’s Refractive Index: 1.756 – 1.804

 

Each gemstone has its own RI, so discovering a sample’s RI can help you figure out what sort of stone it actually is.

 

Step 1 – Place a small bead of RI fluid on the metal surface of the refractometer near the back of the crystal hemicylinder (the window on which the stone will sit).

Step 2 – Place the stone facet face down on the fluid dot and slide it toward the middle of the hemicylinder crystal using your fingers.

Step 3 – Look through the viewer lens without magnification. Continue looking until you see the outline of a bubble, then look at the bottom of this bubble. Take the reading from there, rounding the decimal to the nearest hundredth.

 

Occasionally, you’ll run into the issue of not having a flat surface to work with.  In this instance, you’ll need to leave the top of the refractometer open and hold the rounded stone with your hand.  Hopefully, you can pull a reading off of the gauge.

 

Birefringence Test

Consider testing the birefringence, as well. Birefringence is related to RI. While doing the birefringence test, you will turn the gemstone on the refractometer six times throughout the observation period and note the changes.

Perform a standard RI test. Instead of keeping the stone still, gradually turn it 180 degrees, making each separate turn about 30 degrees. At each 30-degree mark, take a new RI reading.

Subtract the lowest reading from the highest to find the stone’s birefringence. Round it to the nearest thousandth.

Birefringence: 0.046 (max)

 

Single or Double Refraction

Benitoite displays a very low double refraction.

The stone must be transparent for this test to be accurate and beneficial.  If the light won’t pass through the stone, there is no way to test for single or double refraction.

Check for single or double refraction. Use this test on translucent and transparent stones. You can determine whether the stone is only singly refractive (SR) or doubly refractive (DR) to help identify it. Some stones can also be classified as aggregate (AGG).

Turn on the light of a polariscope and place the stone face down on the lower glass lens (polarizer). Look through the top lens (analyzer), turning the top lens until the area around the stone looks darkest. This is your starting point.

Turn the analyzer 360 degrees and watch how the light around the stone changes.

If the stone appears dark and stays dark, it is likely an SR. If the stone starts out light and remains light, it is likely AGG. If the lightness or darkness of the stone changes, it is likely DR.

 

Checking The Diaphaneity

Diaphaneity refers to the mineral’s ability to transmit light. For instance, some minerals are transparent or translucent. A small amount of distortion might occur when they’re thick, but light will pass through them relatively freely.

Benitoite is transparent to translucent.

 

Finding The Specific Gravity

Every stone has its unique specific gravity, which helps us identify them. Specific gravity is one of the best properties to measure when identifying mineral specimens. Most minerals have a narrow range of specific gravity, so getting an accurate measurement can go a long way toward identification.

Specific gravity is a unitless number describing how heavy a mineral is compared to equal volumes of water. For example, if a mineral is three times as dense as water, it’ll have a specific gravity of three. This is useful because while two minerals might be the same size, they’ll each have a different specific gravity.

The larger the sample, the more precise the readings tend to be. Remember that this technique can only be used for single mineral or crystal masses. It will not work for minerals embedded in host rocks.

 

Benitoite’s Specific Gravity: 3.65

 

As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the need for more necessary tools for the job. However, one way to work around this is to hold the specimen and note how heavy or heft it feels compared to what you might expect a specimen of that size to weigh.

If you want to determine the specific gravity of your stone like a pro, you’ll need to invest in a higher-end scale.  The OHAUS Density Determination Kit is the one gemologists use.

 

Identifying Rocks and Minerals Like a Pro

Hopefully, you feel confident in your practice of identifying a piece of Benitoite after reading and applying this guide.  You’ll be using the visual part of this guide the most, and you’ll get better as you interact with more gemstones.  Before you know it, you’ll be identifying stones like a gemologist.

Feel free to reach out if you encounter any issues or need clarification. I’ll do my best to assist you in the identification process.